The present invention relates generally to nonlinear frequency conversion of coherent optical radiation and more particularly to resonant mixing of laser radiation.
Techniques of nonlinear conversion are well known in the art. A major practical drawback of the general nonlinear approach to generating short wavelength coherent radiation is that the conversion efficiency is proportional to the laser power. To obtain useful conversion efficiencies, pulsed lasers are typically used to pump the nonlinear element because of the high peak power these devices produce. Any method which reduces the pump power required to obtain a useful nonlinear conversion would greatly expand the types of lasers which can be used, and therefore would increase the number of available wavelengths.
One alternative method is the buildup cavity or resonant cavity approach, in which the frequency conversion element is surrounded with a buildup cavity wherein the laser power can build up to a value larger than the pump beam power by a factor of many hundred. The net conversion efficiency is improved by the square of this buildup factor.
Specific examples are the frequency doubler in a singly resonant buildup cavity, which have been described in A. Hemmerich et al. [Opt. Lett. Vol. 15, pp. 372-374 (1989), U.S. patent application Ser. No. 07/573,536]; W. J. Kozlovsky et al. [Appl. Phys. Lett. Vol. 56, pp. 2291-2292 (1990)]; L. Goldberg et al. [Appl. Phys. Lett., Vol. 55, pp. 218-220 (1989)]; and G. J. Dixon et al. [Opt. Lett., Vol. 14, pp. 731-733 (1989)].
A dual frequency buildup cavity has been used in frequency doubling by Zimmermann et al. [Opt. Commun. Vol. 71, pp. 229-234 (1989)]. In this experiment, both the pump radiation and the frequency doubled output were resonated to further enhance the doubling efficiency.
Use has been made of the resonant cavity of a laser for intracavity nonlinear conversion, as described for instance by Baer [J. Opt. Soc. Am. Vol. B3, pp. 1175 (1986)] for doubling the laser output, and by Risk et al. [Appl. Phys. Lett. Vol. 52, pp. 85-87 (1988)] for mixing the laser beam with its optical pump beam.
The dual buildup cavity has recently been demonstrated in the laser cavity context by Kean et al., and Grubb et al. [Technical Digest, Conference on Lasers and Electro-Optics, Optical Society of America, Wash. D.C., May 1991, papers CFJ5 and CFJ6]. In these approaches, the laser operates at one of the resonant frequencies, and either the laser pump frequency [Kean, op. cit.] or the doubled output frequency [Grubb, op. cit.] is resonated.
Two lasers have been mixed in a singly resonant buildup cavity by Goldberg et al. [Appl. Phys. Lett. Vol. 56, pp. 2071-2073 (1990)], to produce the desired sum frequency.
It is often desirable to multiply the frequency of a laser by a factor of more than two, e.g., by tripling, or quadrupling. These steps require additional nonlinear conversions. In the prior art, no consideration has been given to the use of a second nonlinear element within the buildup cavity, or to the chaining of successive buildup cavities. What is needed is a buildup cavity configuration which allows multiple frequency conversion steps and which is compatible with the many alternative nonlinear conversion techniques such as doubling, mixing, parametric oscillation, and their combinations.